Okay, subtle vibration. Need diagnostic help.

[leapdragon]

So I recently had a thread about a deep, loud vibration/growl and ended up changing out my engine mounts. I thought the problem was solved, but I spoke to soon. I've just quieted it way down from inside the cabin.

Now the loud, low rumble/growl is gone. The sound is very subtle now and with the new mounts in place I can hear it more clearly in detail, even though it's much quieter. Characteristics:

• Only turns up once I have both (a) laid into the gas a bit and (b) reached at least 40mph or so; if I don't do these things, I can drive for a good long time after starting and then end the trip and it never happens

• Once you've created the noise once (by laying into the gas and reaching at least 40mph or so) it will always appear above 30-40mph and disappear below that speed for the rest of the drive

• Definitely associated with wheel speed, not engine speed

• It's often very subtle but it does get much more prominent if you accelerate hard, then you begin to feel it almost as a rapid thump-thump-thump (though it is still quiet)

• If I put it into neutral at speed, there is no change in neutral; the noise is still there coasting in neutral until you drop below 30mph or so; neutral also doesn't change the sound's frequency (i.e. tied to wheels, not to engine again)

• Breaking, either soft or hard, does not change the noise; it remains unchanged with braking and there is no particular vibration in steering wheel or pedal during breaking

• I don't have any clicking/chirping/grinding noises, and it does not sound like metal/metal or friction/impact, it has more the sound of something very rapidly "flopping around" a bit, though the frequency is very high

• I got under and pulled around on the CV joints, axle shafts, drive (AWD) shaft, etc. everything feels very tight and solid, no ripped boots, no play in anything, no excessive grease anywhere, no in-out play on any side of the angle gear

• Have recently checked wheels for bearing play, none detected

• Note that before replacing engine mounts, it was all quite loud inside the passenger cabin, now it's very quiet (but still there) so it's being transmitted through the frame and so on

• Feels like the sound is "everywhere," hard to isolate any particular direction, corner of the car, etc. with windows open or closed

This seems like it's got to be one of:

• CV axle

• CV axle carrier bearing

• AWD Drive shaft

• AWD Drive shaft carrier bearing

My reasoning for excluding wheel balance: you wouldn't expect it to get louder when you hit the gas hard if it was a wheel balance issue, but in this case, if I rapidly accelerate to 70 mph it's louder during acceleration than if I slowly accelerate to 70 mph.

My reasoning for excluding brake warp: you'd expect something to change when you give it brakes, and/or pedal or steering vibration.

I'm going to get under and check the angle gear for fluid later, I tend to replace its fluid every 15-20k or so, so it's usually fullish and I didn't notice anything off last time I did it.

I'll have some more time to get out there and play later today and then during the week.

Looking for more diagnostic tips and tricks to try to help me figure out where it's coming from.

[jonesg]

what about when the car leans from one side to the other, shifting the load so to speak from side to side. does it favor a side ..?

Wheel bearing, on mine started as a low moan, you can't always feel it or I couldn't until I had the bearing carrier in my hand. I turned the radio up until (10K miles later) it got so bad it was obvious where the problem was. The radio was more annoying.

maybe put it up on stands and run it up.
or have someone drive whilst you probe around the floor with a stethoscope.
you could pull the prop shaft, thats more work but might give more direction.

I would probably just wait it out.... I'm good at that.

[leapdragon]

So far, I can't seem to get it to favor a side. My gut doesn't think it's a wheel bearing, though that doesn't mean much. I can't get it to come through on a recording well (too much cabin noise at speed), BUT—in this YouTube video there is a noise in the background over the first 15 seconds of the video that sounds almost *exactly* like what I get at about 60 mph in the cabin when coasting after a hard acceleration:



It's very subtle/low volume in the cabin at speed when coasting or maintaining, almost undetectable unless you listen carefully, BUT if you step on the gas to accelerate, it gets louder to the point that you can clearly hear it and start to feel it in the center console and seats. And of course, dropping into neutral (letting the engine fall from 3k RPM down to like 650) changes nothing, so that would suggest it's in the driveline somewhere, not in the engine/transmission proper.

Unfortunately it's just me here, not sure who I could get to help out, and I don't have a lift, just jack stands.

Reading forums I've seen a lot of "get your wheels balanced" but my gut says that if it was an out of balance wheel the sound wouldn't change that much due to acceleration (it's a significant change in volume when you step hard on the gas at speed). Same with brakes, my intuition says that if it was a bad caliper or a warped rotor, stepping measurably on the brakes at speed would change the sound or cause an increase in vibration, not no change.

Questions for anyone who is more experienced than me:

• Do CV axles ever go bad with no play and no visual boot damage, or is this basically unheard of? (Both of the fronts here are original with >200k on them.)

• How often does the passenger side carrier bearing for the CV axle go bad? This one seems fine, but it also has >200k. I wonder about this because it's bolted to the block, and so would transmit a lot of vibration if it had failed—though it shows no outward signs of failure...

• Is it actually plausible (or even likely) that an out-of-round or misbalanced wheel would cause a noise like this and that the noise would get measurably louder on acceleration?

• Is it plausible (or even likely) that a warped rotor or bad caliper could cause a noise like this, in such a way that applying brakes doesn't actually change the noise?

• Assuming you hit a significant pothole in early summer... What would be the most likely damage areas that could cause a noise like this?

I'm toying with the idea of just fetching some aftermarket new front axles for cheap and seeing if that changes the sound. I do actually have a parts car here with 150k on it, so I have a spare angle gear, spare OEM axles, etc. but I hate to just start pulling stuff off and moving it over willy-nilly, esp. without knowing whether all the parts on the parts car are good.

One reason I hesitate to just wait on it is that I do mostly highway driving, so any sudden failure would not be... happy.

Also, before changing all the mounts again, this manifested as a loud, low rumble in the passenger compartment that sounded like a muffler delete or something, i.e. it was pretty loud and pretty deep. So something is actually not happy, and now the new mounts are dampening it significantly, but whatever it is, force is def. being applied in ways that will eventually break something.

[leapdragon]

Further thought: doing some math—

if I'm 60mph, with 17" wheels, that's 1056 inches/second. Which works out to around 60Hz with 17" rims. If I generate a 60Hz tone, that's clearly not the right frequency for the 60mph noise I'm getting at 60Hz, much too fast. If I generate 20Hz and 30Hz tones, it's definitely somewhere in that range.

That would indicate something on a 1:2 to 1:3 ratio vs. the wheels OR something (say some kind of joint) that hits a motion extreme twice per revolution as it flops (one side, then the other). That also tends to suggest prop shaft (1:2 on universal joint) or CV axles (1:3 joint structure, no?)

Or am I crazy here?

[Blacklab467]

I would discourage you from trying Chinese axles to establish a good baseline. I put 2 sets of Cardone's in mine and both were undriveable due to vibration and returned to Rock Auto. I've had my original GKN Axles wear out and cause vibration on acceleration without damage to the boot prior. At 200000K miles, it could be your CV joint(s). They would vibrate when they are in the highest usage location axially and continue to vibrate as long as they stay in that position (I'm referring to the CV tripod and hub). I've probably got the nomenclature wrong on these parts but if you can imagine the inside of a CV joint changing position axially inside the hub going over a bump or turning around a corner, there will always be a "most used, most worn" location of the moving parts. Typically this is when the vehicle is under acceleration at a common speed, 45-65 Mph. My experience with CV joints is that they get worse very quickly ( within a couple thousand miles) such that the vehicle becomes almost undriveable, and mostly afflicts the passenger outside joint. It seems to operate at the most extreme angles just from visual observation. I bought new GKN Leobro's for about $325 each.
Interesting that you are suspect of the propshaft also. My car has developed a minor vibration at exactly 83 kmh (50 Mph) and goes away at exactly 97 kmh. I have become suspicious of the propshaft and/or bearing and have harvested a good donor to install and see if I can solve this problem. The propshaft CV's shouldn't wear much but my car has 308000 KMs and yours has about the same at 200000 miles. I'd be interested to know if others have experience with these shafts wearing out at this type of mileage.
Additionally, as another troubleshooting method you could remove the propshaft and the angle gear in separate instances to pinpoint these items as the source of vibration. Neither take more than 2 hours to remove including reassembly of the passenger CV and strut to knuckle and it is safe to drive with either or both removed.

[jonesg]

which wheel hit the pothole.?
thats the side where i'd start, try swapping that front wheel to the rear.
If the noise changes with that wheel on the rear you might have broken belts in the tire,
its balanced but does funny dynamic stuff.

also inspect subframe bushings.

[leapdragon]

Ah, that's a good idea, I should have thought of that. I'll put the fronts on the back and see if anything changes. Thanks!

[leapdragon]

Update:

It was the drive shaft. (AWD prop shaft).

I tried everything else first (replaced all mounts, replaced CV axles, put a boroscope into the angle gear and replaced fluid, rotated tires). The mounts and axles made things somewhat better, but the fundamental issue was still there.

Finally gave in and pulled the shaft. Did it last because this is not my favorite job. At least on my two current XC70s, the exhaust has to come at least partially down to get space to articulate the shaft enough to get it out from between the front and rear flanges. I hate dangling, rusty exhaust stuff when I'm on my back next to ramps and jack stands.

The carrier bearing rubber was ripped clean off he metal and the metal was slightly misformed at the bottom as though the prop shaft hit it. Which sucks because this shaft was just replaced last year.

BUT... vibration is 100% gone driving FWD. The angle gear flange still feels tight and solid, same at the Haldex end.

Also: Watch out for those potholes. After I hit this one in the early summer, I thought all was well, but noise and vibration gradually got worse and worse. The fact that replacing engine mounts and replacing CV joints both improved things somewhat tells me they suffered some as a result, too. Hopefully now I've spotted all the damage.

[jonesg]

wow.